The present invention involves a method of controlling the solids circulation between a downflow reactor and an entrained bed reactor.
In view of recent increases in the price of crude oil, researchers have been searching for alternative sources of energy and hydrocarbons. Much research has focused on recovering the hydrocarbons from hydrocarbon-containing solids such as shale, tar sand or coal by heating or pyrolysis to boil off or liquefy the hydrocarbons trapped in the solid or by reacting the solid with steam, for example, to convert components of solid carbonaceous material into more readily usable gaseous and liquid hydrocarbons. Other known processes involve combustion of the solid carbonaceous materials with an oxygen-containing gas to generate heat. Such processes conventionally employ a treatment zone, e.g., a reaction vessel, in which the solid is heated or reacted.
In a typical coal gasification process, coal is contacted with steam and an oxygen-containing gas to produce a gaseous product.
When air is used as the oxygen-containing gas, the gaseous product contains high levels of nitrogen, which reduces the BTU content of the gaseous product. Some processes have used pure oxygen instead of air, in order to avoid having nitrogen in the gaseous product. This does eliminate the nitrogen from the product but it requires a source of pure oxygen, some oxygen plants are almost as large as the coal gasification plant they are supplying. Thus, one was faced with the alternatives of either produccing a gaseous product diluted with nitrogen or finding a source of pure oxygen for their process.
Another solution to the nitrogen dilution problem is disclosed in U.S. Pat. No. 4,157,245. In one embodiment of the invention disclosed in that patent, a solid heat-transfer material, such as sand, is introduced into an upper portion of a reaction vessel and coal is introduced into a lower portion of the vessel. The physical characteristics of the heat-transfer material and the coal differ such that a superficial velocity of a fluid flowing upwardly through the vessel is greater than the minimum fluidizing velocity of the heat-transfer material and the terminal velocity of the coal, but is less than the terminal velocity of the heat-transfer material. A substantially countercurrent vertical flow of the two solids is maintained in the vessel without substantial top-to-bottom backmixing by passing steam upwardly through the vessel at a rate sufficient to fluidize the heat-transfer material and entrain the coal whereby the heat-transfer material substantially flows downwardly in a fluidized state through the vessel and the coal substantially flows upwardly in an entrained state through the vessel. The steam reacts with the coal to form a hot char and a gaseous product. The heat-transfer material acts as a source of heat for the reaction between the steam and the coal. Cooled heat-transfer material is removed from a lower end of the vessel and the hot char and the gaseous product are removed from an upper end of the vessel. The gaseous product is then separated from the hot char by regular separation techniques.
In one method, the heat-transfer material can be heated by introducing it into an upper portion of a combustion zone, introducing the hot char into a lower portion of the zone, and contacting the heat-transfer material with the hot char while maintaining substantially countercurrent plug flow of the two solids by passing air upwardly through the combustion zone at a rate sufficient to fluidize the heat-transfer material and entrain the char. The heat-transfer material substantially flows downwardly through the combustion zone in a fluidized state and is heated while the char substantially flows upwardly through the combustion zone in an entrained state and is combusted.
The process in U.S. Pat. No. 4,157,245 is based in part on the discovery that in the typical coal gasification process, there are two separate reactions occurring in the same vessel: (1) an endothermic reaction between the coal and steam which produces the gaseous product, and (2) an exothermic reaction between the coal and the oxygen-containing gas which produces the heat necessary for the first reaction. The process of U.S. Pat. No. 4,157,245 separates these two reactions in two separate vessels and transfers the heat generated by the second reaction to the site of the first reaction via a heat-transfer material.
A major advantage of this process is that air can be used as the oxidizing gas without causing the resulting gaseous product to be diluted with nitrogen.
A major disadvantage of this process is that the sand rates must be carefully balanced in various sections of the circulation loop, otherwise the system breaks down.